In one type of offshore well system, a surface wellhead assembly is located at the upper end of a riser that extends downward to a subsea wellhead assembly. The surface wellhead assembly is mounted stationarily on an offshore platform that has legs extending to the seafloor. The riser and surface wellhead assembly are stationary relative to the platform and not subject to wave and current movements.
A casing hanger is supported on a load shoulder of the surface wellhead assembly. The casing hanger supports a string of casing, which has a lower end that latches or ties back to a subsea casing hanger in the subsea wellhead housing at the seafloor. The operator wishes to apply tension to the string of casing to a desired level and to maintain the casing string at that amount of tension. Applying and maintaining the tension may be handled in different ways. For example, a ratcheting mechanism may be employed. The ratcheting mechanism allows part of the casing hanger to ratchet upward relative to the surface wellhead assembly, but not downward so as to hold tension. Alternately, the casing hanger may have an adjustable load shoulder that secures to external threads on the casing hanger. The casing hanger may also have an interior profile for securing to a running tool. While one portion of the running tool pulls upward on the casing hanger to provide the desired amount of tension in the string of casing, another portion of the running tool rotates the adjustable landing shoulder downward into contact with the load shoulder in the surface wellhead assembly.
After the string of casing has been tensioned and the casing hanger landed, the operator would typically remove the running tool, then install a seal or packoff between the outer diameter of the upper portion of casing hanger 23 and the bore in the surface wellhead assembly.
While installing the casing string, it would be advantageous to be able to know the strain and the amount of tension that exists after the casing hanger is set. Also, from time to time it would useful to monitor the strain to determine if the initial tension has decreased, such as might occur if the platform settles. Fatigue can occur due to cycles of stress, either from thermal changes or other factors. Although the surface wellhead assembly is located on a platform above the sea, the casing hanger is concealed within the surface wellhead housing and the casing string within the riser. Consequently, conventionally measuring strain in the same manner as one would to accessible conduits is not possible.